This invention pertains to hoisting devices and particularly to the type of device used for automobile wreckers, in which a cable runs from a drum over at least one sheave which serves to change the direction of the cable before it is effective to lift the weight to which it is attached.
In many auto wrecker hoists and tow cars, particularly those designed for hauling heavier units such as trucks, the principal lifting cable is wound on a drum located close to the cab of the truck on which the unit is mounted. From the drum the cable passes upward in a near vertical direction to a level from which the cable can run nearly horizontally to a sheave from which the cable runs out to the object to be lifted or towed.
It is obviously desirable that the cable lie in even layers on the drum. If the run of cable from the drum to the first sheave is relatively long, the angle at which the cable meets the drum is nearly uniform as the cable is wound onto the drum. In that situation, the cable usually will wind onto the drum reasonably uniformly. However, when the distance between the drum and the first sheave is relatively short, then the angle at which the cable meets the drum can vary considerably. When this happens, there is much less likelihood that the cable will wind onto the drum properly.
In the auto wrecker type of device, the chain ordinarily is at the level of the bed of the truck on which the crane is mounted. From the drum the cable runs upward about to the median level at which the top of the lifting boom might be used. Ordinarily that distance is not great and so the tendency of the cable is to bunch up in the middle of the drum.
Various devices have been used to cure the problem. Among the devices have been idler sheaves each having a variably sloping axle which is designed to tilt back and forth to cause the idler to run back and forth on the axle to cause the cable to wind properly on the drum.
By my invention, I provide a positively actuated idler sheave enclosed in a portion of the structure of the crane. The rotation of the sheave by the cable as it is let out or pulled in causes the sheave to move across its axle and thereby to guide the cable off from or onto the drum. Thus, the device is self driven without the need for any outside actuation to cause it to operate properly.